Actually, I did insert the batteries wrong way after the software crash. My calc crashed and I was unable to turn it on again. I tried all procedures to reset it (even a hole). Inserting the batteries the other way helped to solve the problem. But now I realized that it was stupid decision…

I read that a zener diode is burnt when batteries are inserted in an HP48GX with reversed polarity. I have a crashed (broken LCD, RAM donated to my HP42S+) and I'm taking a look at its components. The negative pole is the chassis, and it has three active contact points that connect the mainboard to the chassis and the negative pole. Positive pole is a tab, firmly soldered to the mainboard.

The HP41 allows batteries to be inserted in reversed order, also clamshells (HP28 and HP18/19). Voyagers and Pioneers don't, because batteries compartment is designed so they do not make contact with input connectors if they are inserted in reverse order.

I have inadvertently inserted batteries in reversed order in my 41's a couple of times without any trouble except for a MEMORY LOST. Also my HP28S suffered a blackout because of distraction. And my HP48SX was once offended by my confused orientation. I once tried to do the same in my HP15C and the last battery simply does not fit inside the batteries compartment.

They are working fine, but now I'm worried about this warning. I'd like knowing a bit more about it, because the only warning that exists in the HP48 manual is that the calculator will not turn on if batteries are inserted in reversed order, not that it will have internal circuits burnt out.

I never heard about it. I'd guess it's a joke, mostly because humidity may condense and cause bad connection on keyboard and other parts of the calculator. There are some specs about storage and daily use temperature ranges, but relative humidity most be taken into account.

there are four SMD diodes easily visible in the HP48G board, two of them are closed to the positive batt pole and the other two are in the mid left of the board, component face.

Which of them is the zenner diode? I tried to follow the coper trails but I do not know how many layers does the board have, so I thought it would be a bit waste of time. And these SMD diodes have nothing but cathode identification.